On Feb 7, 2014, at 12:06 AM, Vishnu Bhatt <vishnu.bhatt@xxxxxxxxxxx> wrote:
> How does wireshark know when to reassemble the TCP segments? How TCP gets to know in wireshark that whether to send the payload to the upper layer or to reassemble it in some other frame?
It gets told by the dissector for the upper layer.
> Is there any header field?
Not in TCP. The service offered by TCP is a byte stream, with no packet boundaries visible; a TCP implementation can divide up the bytes handed to it by the upper-layer protocol in any way that it chooses.
This means that if the protocol running atop TCP has message boundaries, that protocol must somehow arrange to determine when a message begins or ends.
Many protocols do this by putting a message length field into the message header; for example, that's how DNS-over-TCP, ONC RPC-over-TCP, and SMB-over-TCP work. Wireshark has a routine tcp_dissect_pdus(), which a dissector for a protocol running on top of TCP and using a message length field can use.
For HTTP, the header for a request or response continues up to a blank line, and the entity-body for a request or response might be indicated by the connection on which it's being sent closing, by the length of the entity-body being specified in the header with a Content-Length: header field, or in various other fashions.